I think it probably would fixed some the emission annoyances the earlier NA models had, especially with the manual transmission.

I remember 08 Impreza 2.5i 5-Speed Manual would have rev hangs before it would drop, idle would die a little and then the ECU would prop it back up. I think also it might be some of the cause of the low speed bucking I would get.

Compared to my 11 Legacy 2.5i 6-Speed Manual, I don't get rev hangs (just maybe if I wind up 1st gear - more of a heavy flywheel weight), the bucking is very subtle, and idle may dip once in a great while.